As men age, their serum testosterone concentrations decrease, so by age 70-80 years, about 70% of men have values subnormal for young men. Also as men age they develop many other abnormalities, including weakness, frailty and disability; sexual dysfunction; impaired cognition; and loss of vitality and sense of well- being. One reason for thinking the decrease in testosterone could be a cause of the other abnormalities is that young men who develop a low testosterone due to known disease develop similar abnormalities, and these are usually corrected by replacement of testosterone. Testosterone replacement of elderly men, however, has given inconsistent results. The Testosterone and Aging Committee of the Institute of Medicine reviewed the data on testosterone treatment of elderly men and recommended performing a coordinated set of short-term randomized, placebo-controlled efficacy trials of testosterone in each of the four areas above. The overall goal of this application is to develop, in the course of of one year, a protocol for the set of coordinated trials the IOM committee recommended. Toward this goal, we shall pursue these specific aims: 1. Develop a common protocol for all subjects in all trials, including inclusion criteria to ensure that all subjects have a low testosterone and related symptoms; exclusion criteria to protect men at high risk for testosterone-dependent disease; randomized, placebo-controlled design of testosterone treatment; and monitoring for testosterone-dependent diseases. 2. Develop four specific trials in the areas of weakness, frailty and disability; sexual dysfunction; cognitive impairment; and diminished vitality; with entry criteria to include men with abnormalities likely due to low testosterone and exclude men not likely to respond to testosterone. 3. Establish a statistics and data coordinating center to serve the entire set of trials for statististics, project management, core data set, data management plan, data analysis plan, data safety management plan, and development of a clinical research network. 4. Implement aims 1-3 during the planning year by continuing to work within the organizational structure this group of investigators has already established, ie working groups overseen by an executive committee, meeting by conference call and in person and keeping strict deadlines.